I was wondering when the topic of slavery would come up. I am glad Kirstin Samuelian brought it up so we can go at this one too.
A few facts I have garnered from various sources will help us.
First, slavery was outlawed in the British Isles itself in 1772. Famously the Lord Chief Justice Mansfield on 22 June 1772 said in a case which was the result of a pair of abolitionists who helped a slave, James Somersett, from Virginia escape to England. The Virginia owner chased his property down and sued for repossession. Mansfield said the slave in England is not a slave because (quite simply) "pure air" of England and slavery were incompatible. And that was that. Lord Mansfield wrote for the whole bench that "as soon as a slave set his foot on the soil of the British isles he was free" (Encyclopeida Britannica, sub "slavery.") Austen therefore grew up in a world where she didn't personally see slavery. This is not unimportant in gauging the attitude towards slavery in the novel.
The second step in our history comes in 1792 that Wilberforce moved to abolish the slave trade. But all that produced was a political fracas as no law was passed. Step three came in 1807 when the English slave trade was outlawed; however, since there were no effective penalities in the 1807 bill it wasn't actually stopped until 1811 when Brougham's bill made the penalty for trading slaves transportation. And of course slavery itself kept on for a very long time everywhere in the world after that.
Now I am not sure about step four, also important, but I believe it came after Austen died. I am talking here of the law that made trading slaves and ferrying them in English ships not only against the law, but put teeth into the law--if one could catch one of these the prize was great.
How much was known about the horrific conditions of the slaves in the West Indies in England I don't know, but I would like to offer the following unpopular view (which is still held by nowadays quiet Austen scholars) that slavery in Mansfield Park is background, not foreground. In the way the chapel is used to suggest the total lack of moral bearings in the group at Sotherton--left bare, unused, silent, so is the allusion to slavery used to suggest the fatuity and ignorance of each and every member of the younger generation at Mansfield Park--except Fanny who would have liked to ask Sir Thomas a question about the issue (this is later in the book) except everyone else is so bored to tears by any serious subject she drops it. I remember Southam's article very well (see posting on Vol II, Ch 3: "Southam's Article"), and I believe he misread the scene. It's not that everyone is so ashamed; it's that the other young people are bored to tears; Fanny does not repeat her question lest her cousins (except for Edmund) think her a hypocrite as they did when she cried when Sir Thomas first left.
What I'm trying to say is there is no emphasis on slavery as such in Mansfield Park. Rather the allusions to slavery are used emblematically to tell us things about the characters which are not necessarily to tied to any notion that a slaveowner is necessarily evil or corrupt or deeply wrong in his character or values. Far from the more simple interpretation that Sir Thomas is to be taken as corrupt because his wealth is based on slavery, I suggest Austen presents him as a complex real man in a real world; he is a serious person who works hard, and is weary and exhausted and has had severe caustic moral lessons dealing with the whole of his experience in Antigua and is tired, weary, has had what we might call a chance to think and feel, and will gradually become a better man for it when he returns to England. For example (to get a little ahead) when he returns home and has his talk to Mrs Norris about why she permitted the play we are onto the first phase of his eventual total disillusionment with Mrs Norris and realization of what are her real and mostly abysmal motivations and what harm such a person can wreak in little and at large.
Having said this I'd like just to qualify (lest I be misunderstood): if the slavery issue is background, atmosphere, a reinforcement of the novelist's stance that most of the characters who belong to the strata which owns a Mansfield Park or a Sotherton are hollow, unthinking, unknowningly utterly amoral (except for our charming Henry who knows it all right--and Mary too who knows it too, but in a lesser degree because she is less amoral or less aware of her ruthlessness when push comes shove), still background is central, and I would hazard that anything specific related to Antigua in particular, especially the troubles that led Sir Thomas to go out there, and the troubles that kept him there are central to our understanding of Sir Thomas himself as well as reinforcing our general sense of the unjust basis upon which his retreat into a walled-off environment exists. I wonder if anyone can cite what are the facts that are known as to why Sir Thomas had to go to Antigua, what kept him there so long, and what are the possible reasons which permitted him to return to England. The novel events we are now reading are set in 1808 and 1809 and very carefully dovetailed into the calendar for those years.
There are several books which are often cited on this: Kirstin Samuelian referred us to Said's Culture and Imperialism; also often cited are: Maaja Stewart's Domestic Realities and Imperial Fictions, Charles Law, "The Abominable Traffic: MP and the Dynamics of Slavery" in History, Gender and Eighteenth Century Literature (U of Georgia P, l994); Roger Sales's new book, JA & Regency England is said to discuss the issue. But do any of these give any hard specifics about Antigua in the years 1808-9. Further does anyone know what "the strange American business" Dr Grant refers to in a later conversation to Edmund which he has read about in his Quarterly Review (QPcame out 1809 so it does fit).